Binuclear pentafluorocyclopentadienyl metal carbonyls of iron, cobalt, and nickel: Effect of fluorine substitution

Density functional theory studies on the binuclear pentafluorocyclopentadienyl metal carbonyls (C5F5)2M2(CO)n (M = Fe, n = 4, 3, 2; M = Co, n = 3, 2, 1; M = Ni, n = 2, 1) using the B3LYP and BP86 methods find pentahapto η5-C5F5 ligands in all of the low energy structures. In general, the low energy structures of these η5-C5F5 derivatives parallel rather closely those of their hydrocarbon analogs with relatively little change upon complete substitution of the hydrogen atoms with fluorine atoms. The lowest energy (C5F5)2M2(CO)n (M = Fe, n = 4; M = Co, n = 3; M = Ni, n = 2) structures are predicted to be closed-shell singlets with metal–metal distances of ∼2.6 Å, ∼2.5 Å, and ∼2.4 Å, respectively, corresponding to the single bonds required to give each metal atoms the favored 18-electron configuration and similar to their experimentally known hydrocarbon analogs. For (C5F5)2Fe2(μ-CO)2(CO)2 the cis and trans isomers are predicted to have similar energies within 2 kcal/mol similar to the well-known (C5H5)2Fe2(μ-CO)2(CO)2 except the relative energies of the cis and trans isomers are reversed. For the unsaturated derivatives (C5F5)2M2(CO)n (M = Fe, n = 3, 2; M = Co, n = 2, 1; M = Ni, n = 1), the lowest energy structures have shorter metal–metal multiple bonds of orders 2 and 3 corresponding to the favored 18-electron configurations for the metal atoms.